Report unsupported ALTER CONSTRAINT explicitly

[ibrahim halatci (ihalatci)]

Draft fix for #8758 after the PostgreSQL 19 Beta 4 reassessment.

Beta 4 reverted partition MERGE/SPLIT, so this PR should contain only the retained ALTER TABLE ... ALTER CONSTRAINT guardrail:

• return a stable FEATURE_NOT_SUPPORTED error for distributed, reference, and Citus-local tables;
• explain that constraint property changes are not propagated to shards;
• preserve ordinary PostgreSQL local-table behavior;
• cover PG19 CHECK constraint enforceability and supported-version rejection behavior.

Before review, rebase onto current main and remove unrelated DETACH PARTITION error-code changes unless they are split into a separately justified PR.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.77%. Comparing base (991faa8) to head (eaacf5e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8808      +/-   ##
==========================================
- Coverage   88.77%   88.77%   -0.01%     
==========================================
  Files         290      290              
  Lines       65132    65134       +2     
  Branches     8223     8222       -1     
==========================================
+ Hits        57823    57824       +1     
- Misses       4936     4937       +1     
  Partials     2373     2373              

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